School of Computing Science Simon Fraser University

1
School of Computing Science Simon Fraser
University

• CMPT 371 Data Communications and Networking
• Chapter 5 Data Link Layer

2
Chapter 5 The Data Link Layer

• Our goals
• understand principles behind data link layer
  services
• error detection, correction
• sharing a broadcast channel multiple access
• link layer addressing
• reliable data transfer, flow control done!
• implementation of various link layer technologies

3
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM and MPLS

4
Link Layer Introduction

• Some terminology
• hosts and routers are nodes
• communication channels that connect adjacent
  nodes along communication path are links
• wired links
• wireless links
• LANs
• layer-2 packet is a frame, encapsulates datagram

data-link layer has responsibility of
transferring datagram from one node to adjacent
node over a link
5
Link layer context

• transportation analogy
• trip from Burnaby to Lausanne, Switzerland
• limo Burnaby to YVR
• plane YVR to Geneva
• train Geneva to Lausanne
• tourist datagram
• transport segment communication link
• transportation mode link layer protocol
• travel agent routing algorithm

• Datagram transferred by different link protocols
  over different links
• e.g., Ethernet on first link, frame relay on
  intermediate links, 802.11 on last link
• Each link protocol provides different services
• e.g., may or may not provide rdt over link

6
Link Layer Services

• Framing, link access
• encapsulate datagram into frame, adding header,
  trailer
• channel access if shared medium
• MAC addresses used in frame headers to identify
  source, dest
• different from IP address!
• Reliable delivery between adjacent nodes
• we learned how to do this already (chapter 3)!
• seldom used on low bit error link (fiber, some
  twisted pair)
• wireless links high error rates
• Q why both link-level and end-end reliability?
• LL local correction (bet adjacent nodes) ?
  faster
• e-2-e is still needed because not all LL
  protocols provide reliability

7
Link Layer Services (more)

• Flow Control
• pacing between adjacent sending and receiving
  nodes
• Error Detection
• errors caused by signal attenuation, noise
• receiver detects presence of errors
• signals sender for retransmission or drops frame
• Error Correction
• receiver identifies and corrects bit error(s)
  without resorting to retransmission
• Half-duplex and full-duplex
• with half duplex, nodes at both ends of link can
  transmit, but not at same time

8
Adaptors Communicating
datagram
rcving node
link layer protocol
sending node
adapter
adapter

• receiving side
• looks for errors, rdt, flow control, etc
• extracts datagram, passes to rcving node
• adapter is semi-autonomous
• link physical layers

• link layer implemented in adaptor (aka NIC)
• Ethernet card, PCMCI card, 802.11 card
• sending side
• encapsulates datagram in a frame
• adds error checking bits, rdt, flow control, etc.

9
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

10
Error Detection

• EDC Error Detection and Correction bits
  (redundancy)
• D Data protected by error checking, may
  include header fields
• Error detection is not 100 reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and
  correction

11
Error Detection Parity Checking
Two Dimensional Bit Parity Detect and correct
single bit errors
Single Bit Parity Detect single bit errors
1
Even parity make number of bits that are 1 even
0
0
Need to detect more errors with fewer bits
12
Internet checksum

• Goal detect errors (e.g., flipped bits) in
  transmitted segment (note used at transport
  layer only)

• Receiver
• compute checksum of received segment
• check if computed checksum equals checksum field
  value
• NO - error detected
• YES - no error detected. But maybe errors
  nonetheless

• Sender
• treat segment contents as sequence of 16-bit
  integers
• checksum addition (1s complement sum) of
  segment contents
• sender puts checksum value into UDP checksum
  field

13
Cyclic Redundancy Check (CRC)

• Goal Maximize probability of detecting errors
  using a small number of redundant bits
• E .g., CRC-32 used in many protocols
• uses only 32 bits and detects most of errors in
  messages thousands of bytes long
• CRC is based on a branch of mathematics called
  Finite Fields
• We will cover only the basic ideas here
• Represent a message D of length d1 bits as a
  polynomial of degree d
• 10011010 ? D(x) x7 0 0 x4 x3 0 x1
  0 x7 x4 x3
  x1

14
CRC basic idea

• Sender and receiver agree on a divisor polynomial
  G(x) of degree r
• Sender transmits T(x), which consists of d1
  data bits AND r redundant bits such that
  G(x)T(x),
• i.e., the remainder of dividing T(x) by G(x) is 0
• Receiver gets T(x) which may have corrupted
  bits
• If G(x) T(x) then no errors occurred

15
CRC

• Notes on polynomial arithmetic modulo-2
• Any B(x) can be divided by C(x) if degree
  of B(x) gt degree of C(x)
• Remainder of B(x)/C(x) is obtained by subtracting
  C(x) from B(x)
• Subtraction (and addition) is the same as XORing

16
CRC

• How does sender make G(x) T(x)?
• Multiply D(x) by xr let T(x) be the result //
  performed as shift left r bits
• Divide T(x) by G(x) and find remainder
• Subtract remainder from T(x) //
  performed as XOR

17
CRC Example

• msg D 101110
• G 1001
• R, T?
• Let us work it out
• R 011
• T 101110 011

18
CRC

• How can we choose G(x)?
• Let E(x) be errors added to message ?
• Received message is T(x) E(x)
• Need G(x) that is unlikely divides T(x) E(x)
• We know G(x) T(x) ? we need G(x) that is
  unlikely to divide E(x) for common types of
  errors
• Example common error single bit error ? E(x)
  xi
• How would you choose G(x) to detect that error?
• Set first and last bit of G(x) to be 1 ? E(x)
  G(x) ? 0 for any position I

19
CRC

• Widely used G(x) functions can handle many other
  types of errors
• Burst of errors lt r
• Any odd number of errors
• All double-bit errors
• CRC is efficiently implemented in hardware

20
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

21
Multiple Access Links and Protocols

• Two types of links
• point-to-point
• Single sender and single receiver
• E.g., dial-up links ? point-to-point protocol
  (PPP)
• broadcast (shared wire or medium)
• Multiple senders and multiple receivers
• E.g., traditional Ethernet, 802.11 wireless LAN
• ? need Multiple Access protocol (MAC)

22
Multiple Access protocols

• Two or more simultaneous transmissions on a
  shared channel ? interference (collision)
• Collision node receives two or more signals at
  the same time
• Multiple Access (MAC) protocol
• distributed algorithm that determines how nodes
  share channel, i.e., determine when node can
  transmit
• communication about channel sharing must use
  channel itself!
• no out-of-band channel for coordination

23
Ideal Multiple Access Protocol

• Broadcast channel of rate R bps
• 1. When one node wants to transmit, it can send
  at rate R
• 2. When M nodes want to transmit, each can send
  at average rate R/M
• 3. Fully decentralized
• no special node to coordinate transmissions
• no synchronization of clocks in nodes
• 4. Simple to implement

24
MAC Protocols a taxonomy

• Three broad classes
• Channel Partitioning
• divide channel into smaller pieces (time slots,
  frequency, code)
• allocate piece to node for exclusive use
• Random Access
• channel not divided, allow collisions
• recover from collisions
• Taking turns
• Nodes take turns, but nodes with more to send can
  take longer turns

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Channel Partitioning MAC protocols TDMA

• TDMA time division multiple access
• access to channel in "rounds"
• each node gets fixed length slot in each round
• Slot length pkt trans time
• example 6-node LAN
• Nodes 1,3,4 have pkt, slots 2,5,6 idle
• Cons unused slots go idle ? channel
  under-utilization in light load scenarios

26
Channel Partitioning MAC protocols FDMA

• FDMA frequency division multiple access
• channel spectrum divided into frequency bands
• each station assigned fixed frequency band
• unused transmission time in frequency bands go
  idle
• example 6-station LAN, 1,3,4 have pkt, frequency
  bands 2,5,6 idle

time
frequency bands
27
Random Access Protocols

• When node has packet to send
• transmit at full channel data rate R
• no a priori coordination among nodes
• two or more transmitting nodes ? collision
• random access MAC protocol specifies
• how to detect collisions
• how to recover from collisions (e.g., via delayed
  retransmissions)
• Examples of random access MAC protocols
• slotted ALOHA
• ALOHA
• CSMA, CSMA/CD, CSMA/CA

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Slotted ALOHA

• Assumptions
• all frames same size
• time is divided into equal size slots, time to
  transmit 1 frame
• nodes start to transmit frames only at beginning
  of slots
• nodes are synchronized
• if 2 or more nodes transmit in slot, all nodes
  detect collision

• Operation
• when node obtains fresh frame, it transmits in
  next slot
• no collision, node can send new frame in next
  slot
• if collision, node retransmits frame in each
  subsequent slot with prob. p until success

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Slotted ALOHA

• Pros
• single active node can continuously transmit at
  full rate of channel
• highly decentralized only slots in nodes need to
  be in sync
• very simple

• Cons
• collisions, wasting slots
• idle slots
• clock synchronization

30
Efficiency of Slotted Aloha

• Efficiency (E) is the fraction of successful
  slots
• in the long run, when there are many nodes, each
  with many frames to send
• Suppose N nodes, each transmits in a slot with
  probability p. Determine max efficiency E.
• Let us work it out!
• prob that a given node (say 1) has success in a
  slot p(1-p)N-1
• prob that any node has a success E N
  p(1-p)N-1
• Find p that maximizes E Np(1-p)N-1
• Differentiate ? p 1/N ? E (1 - 1/N)N-1
• As N ? 8, E lt 1/e 0.37
• Channel used for useful transmissions lt 37 of
  time!

31
Pure (unslotted) ALOHA

• unslotted Aloha simpler, no synchronization
• when frame first arrives
• transmit immediately
• collision probability increases
• frame sent at t0 collides with other frames sent
  in t0-1,t01

32
Pure Aloha efficiency

• P(success by given node) P(node transmits) x
• P(no other node
  transmits in t0-1,t0 x
• P(no other node
  transmits in t0,t01
• p (1-p)N-1 (1-p)N-1
• p (1-p)2(N-1)
• E N p (1-p)2(N-1)
• choosing optimum p and letting N ? 8, we get
• E 1/(2e) 0.18
• Even worse!

33
CSMA (Carrier Sense Multiple Access)

• CSMA listen before transmit
• If channel sensed idle transmit entire frame
• If channel sensed busy, defer transmission
• Human analogy dont interrupt others!

34
CSMA collisions
spatial layout of nodes
collisions can still occur propagation delay
means two nodes may not hear each others
transmission
collision entire packet transmission time wasted
note role of distance propagation delay in
determining collision probability
35
CSMA/CD (Collision Detection)

• CSMA/CD carrier sensing, deferral as in CSMA
• collisions detected within short time
• colliding transmissions aborted, reducing channel
  wastage
• collision detection
• easy in wired LANs measure signal strengths,
  compare transmitted, received signals
• difficult in wireless LANs receiver shut off
  while transmitting

36
CSMA/CD collision detection
37
Taking Turns MAC protocols

• channel partitioning MAC protocols
• share channel efficiently and fairly at high load
• inefficient at low load delay in channel access,
  1/N bandwidth allocated even if only 1 active
  node!
• Random access MAC protocols
• efficient at low load single node can fully
  utilize channel
• high load collision overhead
• taking turns protocols
• look for best of both worlds!

38
Taking Turns MAC protocols

• Token passing
• control token passed from one node to next
  sequentially
• node gets token, sends a msg
• concerns
• token overhead
• latency
• single point of failure (token)

• Polling
• master node invites slave nodes to transmit in
  turn
• concerns
• polling overhead
• latency
• single point of failure (master)

39
Summary of MAC protocols

• What do you do with a shared media?
• Channel Partitioning, by time, frequency or code
• Time Division, Frequency Division
• Random partitioning (dynamic),
• ALOHA, S-ALOHA, CSMA, CSMA/CD
• carrier sensing easy in some technologies
  (wire), hard in others (wireless)
• CSMA/CD used in Ethernet
• CSMA/CA used in 802.11
• Taking Turns
• polling from a central site, token passing

40
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

41
MAC Addresses

• 32-bit IP address
• network-layer address
• used to get datagram to destination IP subnet
• MAC (or LAN or physical or Ethernet) address
• used to get frame from one interface to another
  physically-connected interface (same network)
• 48 bit MAC address (for most LANs) burned in the
  adapter ROM

42
MAC Address
Each adapter on LAN has unique LAN address
Broadcast address FF-FF-FF-FF-FF-FF
adapter
43
MAC Address (more)

• MAC address allocation administered by IEEE
• manufacturer buys portion of MAC address space
  (to assure uniqueness)
• Analogy
• (a) MAC address like Social Insurance
  Number
• (b) IP address like postal address
• MAC flat address ? portability
• can move LAN card from one LAN to another
• IP hierarchical address ? NOT portable
• depends on IP subnet to which node is attached

44
MAC and IP addresses

• Why do we have TWO addresses (IP,MAC)? Do we have
  to have MAC addresses?
• Yes, we must have both
• To allow different network-layer protocols over
  same card (e.g., IP, Novell IPX, DECnet)
• Enable flexibility, mobility of cards
• Efficiency imagine that nodes have only IP
  addresses ? ALL packets sent over LAN will be
  forwarded by NIC to the IP layer ? too many
  useless interrupts

45
ARP Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP table
• ARP Table IP/MAC address mappings for some LAN
  nodes
• lt IP address MAC address TTLgt
• TTL (Time To Live) time after which address
  mapping will be forgotten (typically 20 min)

237.196.7.78
1A-2F-BB-76-09-AD
237.196.7.23
237.196.7.14
LAN
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
237.196.7.88
46
ARP protocol Same LAN (network)

• A wants to send datagram to B, and Bs MAC
  address not in As ARP table.
• A broadcasts ARP query packet, containing B's IP
  address
• Dest MAC address FF-FF-FF-FF-FF-FF
• all machines on LAN receive ARP query
• B receives ARP packet, replies to A with its
  (B's) MAC address
• frame sent to As MAC address (unicast)

• A caches (saves) IP-to-MAC address pair in its
  ARP table until information becomes old (times
  out)
• soft state information that times out (goes
  away) unless refreshed
• ARP is plug-and-play
• nodes create their ARP tables without
  intervention from net administrator

47
Routing to another LAN

• walkthrough send datagram from A to B via R
• assume A knows Bs IP
  address
• Two ARP tables in router R, one for each IP
  network (LAN)
• In routing table at source Host, find router
  111.111.111.110
• In ARP table at source, find MAC address
  E6-E9-00-17-BB-4B, etc

A
R
B
48
Routing to another LAN (contd)

• Detailed steps
• A creates datagram with source A, destination B
• A uses ARP to get Rs MAC address for
  111.111.111.110
• A creates link-layer frame with R's MAC address
  as dest, frame contains A-to-B IP datagram
• As adapter sends frame
• Rs adapter receives frame
• R removes IP datagram from Ethernet frame, sees
  its destined to B
• R uses ARP to get Bs MAC address
• R creates frame containing A-to-B IP datagram
  sends to B

49
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

50
Ethernet

• dominant wired LAN technology
• cheap 20 for 100Mbs!
• first widely used LAN technology
• Simpler, cheaper than token LANs and ATM
• Kept up with speed race 10 Mbps 10 Gbps

Metcalfes Ethernet sketch
51
Star topology

• Bus topology popular through mid 90s
• Now star topology prevails
• Connection choices hub or switch (more later)

hub or switch
52
Ethernet Frame Structure

• Sending adapter encapsulates IP datagram (or
  other network layer protocol packet) in Ethernet
  frame
• Preamble
• 7 bytes with pattern 10101010 followed by one
  byte with pattern 10101011
• used to synchronize receiver, sender clock rates

53
Ethernet Frame Structure (more)

• Addresses 6 bytes
• if adapter receives frame with matching
  destination address, or with broadcast address
  (e.g., ARP packet), it passes data in frame to
  net-layer protocol
• otherwise, adapter discards frame
• Type indicates the higher layer protocol (mostly
  IP but others may be supported such as Novell IPX
  and AppleTalk)
• CRC checked at receiver, if error is detected,
  the frame is simply dropped

54
Unreliable, connectionless service

• Connectionless No handshaking between sending
  and receiving adapter.
• Unreliable receiving adapter doesnt send acks
  or nacks to sending adapter
• stream of datagrams passed to network layer can
  have gaps
• gaps will be filled if app is using TCP
• otherwise, app will see the gaps

55
Ethernet uses CSMA/CD

• No slots
• adapter doesnt transmit if it senses that some
  other adapter is transmitting, that is, carrier
  sense
• transmitting adapter aborts when it senses that
  another adapter is transmitting, that is,
  collision detection

• Before attempting a retransmission, adapter waits
  a random time, that is, random access

56
Ethernet CSMA/CD algorithm

• 1. Adaptor receives datagram from net layer
  creates frame
• 2. If adapter senses channel idle, it starts to
  transmit frame. If it senses channel busy, waits
  until channel idle and then transmits
• 3. If adapter transmits entire frame without
  detecting another transmission, the adapter is
  done with frame !

• 4. If adapter detects another transmission while
  transmitting, aborts and sends jam signal
• 5. After aborting, adapter enters exponential
  backoff after the mth collision, adapter chooses
  a K at random from 0,1,2,,2m-1. Adapter waits
  K?512 bit times and returns to Step 2

57
Ethernets CSMA/CD (more)

• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• Bit time 0.1 microsec for 10 Mbps Ethernet for
  K1023, wait time is about 50 msec

• Exponential Backoff
• Goal adapt retransmission attempts to estimated
  current load
• heavy load random wait will be longer
• first collision choose K from 0,1 delay is K?
  512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten collisions, choose K from
  0,1,2,3,4,,1023

See/interact with Java applet on AWL Web
site highly recommended !
58
CSMA/CD efficiency

• Tprop max prop between 2 nodes in LAN
• ttrans time to transmit max-size frame
• Efficiency goes to 1 as tprop goes to 0
• Goes to 1 as ttrans goes to infinity
• Much better than ALOHA, but still decentralized,
  simple, and cheap

59
10BaseT and 100BaseT

• 10/100 Mbps rate latter called fast ethernet
• T stands for Twisted Pair
• Nodes connect to a hub star topology 100 m
  max distance between nodes and hub

60
Hubs

• Hubs are essentially physical-layer repeaters
• bits coming from one link go out all other links
• at the same rate
• no frame buffering
• no CSMA/CD at hub adapters detect collisions
• provides net management functionality

61
Manchester encoding

• Used in 10BaseT
• Each bit has a transition
• Allows clocks in sending and receiving nodes to
  synchronize to each other
• no need for a centralized, global clock among
  nodes!
• Hey, this is physical-layer stuff!

62
Gbit Ethernet

• uses standard Ethernet frame format
• allows for point-to-point links and shared
  broadcast channels
• in shared mode, CSMA/CD is used short distances
  between nodes required for efficiency
• uses hubs, called here Buffered Distributors
• Full-Duplex at 1 Gbps for point-to-point links
• 10 Gbps now !

63
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Interconnections Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

64
Interconnecting with hubs

• Backbone hub interconnects LAN segments
• Extends max distance between nodes
• But individual segment collision domains become
  one large collision domain
• Cant interconnect 10BaseT 100BaseT

hub
hub
hub
hub
65
Switch

• Link layer device
• stores and forwards Ethernet frames
• examines frame header and selectively forwards
  frame based on MAC dest address
• when frame is to be forwarded on segment, uses
  CSMA/CD to access segment
• transparent
• hosts are unaware of presence of switches
• plug-and-play, self-learning
• switches do not need to be configured

66
Forwarding
1
3
2

• How to determine onto which LAN segment to
  forward frame?
• Looks like a routing problem...

67
Self learning

• A switch has a switch table
• entry in switch table
• (MAC Address, Interface, Time Stamp)
• stale entries in table dropped (TTL can be 60
  min)
• switch learns which hosts can be reached through
  which interfaces
• when frame received, switch learns location of
  sender incoming LAN segment
• records sender/location pair in switch table

68
Filtering/Forwarding

• When switch receives a frame
• index switch table using MAC dest address
• if entry found for destinationthen
• if dest on segment from which frame arrived
  then drop the frame
• else forward the frame on interface
  indicated
• else flood

forward on all but the interface on which the
frame arrived
69
Switch example

• Suppose C sends frame to D

address
interface
switch
1
A B E G
1 1 2 3
3
2
hub
hub
hub
A
I
F
D
G
B
C
H
E

• Switch receives frame from C destined to D
• notes in switch table that C is on interface 1
• because D is not in table, switch forwards frame
  into interfaces 2 and 3
• frame received by D

70
Switch example

• Suppose D replies back with frame to C.

address
interface
switch
A B E G C
1 1 2 3 1
hub
hub
hub
A
I
F
D
G
B
C
H
E

• Switch receives frame from D destined to C
• notes in bridge table that D is on interface 2
• because C is in table, switch forwards frame only
  to interface 1
• frame received by C

71
Switch traffic isolation

• switch installation breaks subnet into LAN
  segments
• switch filters packets
• same-LAN-segment frames not usually forwarded
  onto other LAN segments
• segments become separate collision domains

collision domain
collision domain
collision domain
72
Switches dedicated access

• Switch with many interfaces
• Hosts have direct connection to switch
• No collisions full duplex
• Switching A-to-A and B-to-B simultaneously, no
  collisions

A
C
B
switch
C
B
A
73
More on Switches

• cut-through switching frame forwarded from input
  to output port without first collecting entire
  frame
• slight reduction in latency
• combinations of shared/dedicated, 10/100/1000
  Mbps interfaces

74
Institutional network
mail server
to external network
web server
router
switch
IP subnet
hub
hub
hub
75
Switches vs. Routers

• both store-and-forward devices
• Routers network layer devices
• Switches link layer devices ? faster processing
• Routers maintain routing tables, implement
  routing algorithms
• handle complex topologies, find efficient paths
• Switches maintain switch tables, implement
  learning algorithms
• handle simpler (spanning tree) topologies, paths
  may not be optimal

76
Summary comparison
77
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

78
Point to Point Data Link Control

• one sender, one receiver, one link easier than
  broadcast link
• no Media Access Control
• no need for explicit MAC addressing
• e.g., dialup link, ISDN line
• popular point-to-point DLC protocols
• PPP (point-to-point protocol)
• HDLC High level data link control

79
PPP Design Requirements RFC 1557

• packet framing encapsulation of network-layer
  datagram in data link frame
• carry network layer data of any network layer
  protocol (not just IP) at same time
• ability to demultiplex upwards
• bit transparency must carry any bit pattern in
  the data field
• error detection (no correction)
• connection liveness detect, signal link failure
  to network layer
• network layer address negotiation endpoint can
  learn/configure each others network address

80
PPP non-requirements

• no error correction/recovery
• no flow control
• out of order delivery OK
• no need to support multipoint links (e.g.,
  polling)

Error recovery, flow control, data re-ordering
all relegated to higher layers!
81
PPP Data Frame

• Flag delimiter (framing)
• Address does nothing (only one option)
• Control does nothing in the future possible
  multiple control fields
• Protocol upper layer protocol to which frame
  delivered (eg, PPP-LCP, IP, IPCP, etc)

82
PPP Data Frame

• info upper layer data being carried
• check cyclic redundancy check for error
  detection

83
Byte Stuffing

• data transparency requirement data field must
  be allowed to include flag pattern lt01111110gt
• Q is received lt01111110gt data or flag?
• Sender adds (stuffs) extra lt 01111110gt byte
  after each lt 01111110gt data byte
• Receiver
• two 01111110 bytes in a row discard first byte,
  continue data reception
• single 01111110 flag byte

84
Byte Stuffing
flag byte pattern in data to send
flag byte pattern plus stuffed byte in
transmitted data
85
PPP Data Control Protocol

• Before exchanging network-layer data, data link
  peers must
• configure PPP link (max. frame length,
  authentication)
• learn/configure network
• layer information
• for IP carry IP Control Protocol (IPCP) msgs
  (protocol field 8021) to configure/learn IP
  address

86
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM and MPLS

87
Virtualization of networks

• Virtualization of resources a powerful
  abstraction in systems engineering
• computing examples virtual memory, virtual
  devices
• Virtual machines e.g., java
• IBM VM os from 1960s/70s
• layering of abstractions dont sweat the details
  of the lower layer, only deal with lower layers
  abstractly

88
The Internet virtualizing networks

• 1974 multiple unconnected nets
• ARPAnet
• data-over-cable networks
• packet satellite network (Aloha)
• packet radio network

• differing in
• addressing conventions
• packet formats
• error recovery
• routing

satellite net
ARPAnet
"A Protocol for Packet Network Intercommunication"
, V. Cerf, R. Kahn, IEEE Transactions on
Communications, May, 1974, pp. 637-648.
89
The Internet virtualizing networks

• Gateway
• embed internetwork packets in local packet
  format or extract them
• route (at internetwork level) to next gateway

gateway
satellite net
ARPAnet
90
Cerf Kahns Internetwork Architecture

• What is virtualized?
• two layers of addressing internetwork and local
  network
• new layer (IP) makes everything homogeneous at
  internetwork layer
• underlying local network technology
• cable
• satellite
• 56K telephone modem
• today ATM, MPLS
• invisible at internetwork layer. Looks
  like a link layer technology to IP!

91
ATM and MPLS

• ATM, MPLS separate networks in their own right
• different service models, addressing, routing
  from Internet
• viewed by Internet as logical link connecting IP
  routers
• just like dialup link is really part of separate
  network (telephone network)
• ATM, MPSL of technical interest in their own
  right

92
Asynchronous Transfer Mode ATM

• 1990s/00 standard for high-speed (155Mbps to 622
  Mbps and higher) Broadband Integrated Service
  Digital Network architecture
• Goal integrated, end-end transport of voice,
  video, data
• meeting timing/QoS requirements of voice, video
  (versus Internet best-effort model)
• next generation telephony technical roots in
  telephone world
• packet-switching (fixed length packets, called
  cells) using virtual circuits

93
ATM architecture

• adaptation layer only at edge of ATM network
• data segmentation/reassembly
• roughly analogous to Internet transport layer
• ATM layer network layer
• cell switching, routing
• physical layer

94
ATM Adaptation Layer (AAL)

• ATM Adaptation Layer (AAL) adapts upper layers
  (IP or native ATM applications) to ATM layer
  below
• AAL present only in end systems, not in switches
• AAL layer segment (header/trailer fields, data)
  fragmented across multiple ATM cells
• analogy TCP segment in many IP packets

95
ATM Adaptation Layer (AAL) more

• Different versions of AAL layers, depending on
  ATM service class
• AAL1 for CBR (Constant Bit Rate) services, e.g.
  circuit emulation
• AAL2 for VBR (Variable Bit Rate) services, e.g.,
  MPEG video
• AAL5 for data (eg, IP datagrams)

User data
AAL PDU
ATM cell
96
ATM Layer

• Service transport cells across ATM network
• analogous to IP network layer
• very different services than IP network layer

Guarantees ?
Network Architecture Internet ATM ATM ATM ATM
Service Model best effort CBR VBR ABR UBR
Congestion feedback no (inferred via
loss) no congestion no congestion yes no
Bandwidth none constant rate guaranteed rate gua
ranteed minimum none
Loss no yes yes no no
Order no yes yes yes yes
Timing no yes yes no no
97
ATM Layer Virtual Circuits

• VC transport cells carried on VC from source to
  dest
• call setup, teardown for each call before data
  can flow
• each packet carries VC identifier (not
  destination ID)
• every switch on source-dest path maintain state
  for each passing connection
• link,switch resources (bandwidth, buffers) may be
  allocated to VC to get circuit-like perf.
• Permanent VCs (PVCs)
• long lasting connections
• typically permanent route between to IP
  routers
• Switched VCs (SVC)
• dynamically set up on per-call basis

98
ATM VCs

• Advantages of ATM VC approach
• QoS performance guarantee for connection mapped
  to VC (bandwidth, delay, delay jitter)
• Drawbacks of ATM VC approach
• Inefficient support of datagram traffic
• one PVC between each source/dest pair) does not
  scale (N2 connections needed)
• SVC introduces call setup latency, processing
  overhead for short lived connections

99
ATM Layer ATM cell

• 5-byte ATM cell header
• 48-byte payload
• Why? small payload -gt short cell-creation delay
  for digitized voice
• halfway between 32 and 64 (compromise!)

Cell header
Cell format
100
ATM cell header

• VCI virtual channel ID
• will change from link to link thru net
• PT Payload type (e.g. RM cell versus data cell)
• CLP Cell Loss Priority bit
• CLP 1 implies low priority cell, can be
  discarded if congestion
• HEC Header Error Checksum
• cyclic redundancy check

101
ATM Physical Layer (more)

• Two pieces (sublayers) of physical layer
• Transmission Convergence Sublayer (TCS) adapts
  ATM layer above to PMD sublayer below
• Physical Medium Dependent (PMD) depends on
  physical medium being used
• TCS Functions
• Header checksum generation 8 bits CRC
• Cell delineation
• With unstructured PMD sublayer, transmission of
  idle cells when no data cells to send

102
ATM Physical Layer

• Physical Medium Dependent (PMD) sublayer
• SONET/SDH transmission frame structure (like a
  container carrying bits)
• bit synchronization
• bandwidth partitions (TDM)
• several speeds OC3 155.52 Mbps OC12 622.08
  Mbps OC48 2.45 Gbps, OC192 9.6 Gbps
• TI/T3 transmission frame structure (old
  telephone hierarchy) 1.5 Mbps/ 45 Mbps
• unstructured just cells (busy/idle)

103
ATM network or link layer?

• Vision end-to-end transport ATM from desktop
  to desktop
• ATM is a network technology
• Reality used to connect IP backbone routers
• IP over ATM
• ATM as switched link layer, connecting IP routers

IP network
ATM network
104
IP-Over-ATM

• IP over ATM
• replace network (e.g., LAN segment) with ATM
  network
• ATM addresses, IP addresses

• Classic IP only
• 3 networks (e.g., LAN segments)
• MAC (802.3) and IP addresses

ATM network
Ethernet LANs
Ethernet LANs
105
IP-Over-ATM
106
Datagram Journey in IP-over-ATM Network

• at Source Host
• IP layer maps between IP, ATM dest address (using
  ATMARP)
• passes datagram to AAL
• AAL encapsulates data, segments cells, passes to
  ATM layer
• ATM network moves cell along VC to destination
• at Destination Host
• AAL5 reassembles cells into original datagram
• if CRC OK, datagram is passed to IP

107
IP-Over-ATM

• Issues
• IP datagrams into ATM AAL5 PDUs
• from IP addresses to ATM addresses
• just like IP addresses to 802.3 MAC addresses!
• ATM ARP

ATM network
Ethernet LANs
108
Multiprotocol label switching (MPLS)

• initial goal speed up IP forwarding by using
  fixed length label (instead of IP address) to do
  forwarding
• borrowing ideas from Virtual Circuit (VC)
  approach
• but IP datagram still keeps IP address!

PPP or Ethernet header
IP header
remainder of link-layer frame
MPLS header
label
Exp
S
TTL
5
20
3
1
109
MPLS capable routers

• a.k.a. label-switched router
• forwards packets to outgoing interface based only
  on label value (dont inspect IP address)
• MPLS forwarding table distinct from IP forwarding
  tables
• signaling protocol needed to set up forwarding
• RSVP-TE
• forwarding possible along paths that IP alone
  would not allow (e.g., source-specific routing)
  !!
• use MPLS for traffic engineering
• must co-exist with IP-only routers

110
MPLS forwarding tables
in out out label
label dest interface
10 A 0
12 D 0
8 A 1
R6
0
0
D
1
1
R3
R4
R5
0
0
A
R2
R1
111
Chapter 5 Summary

• data link layer services
• error detection, correction
• sharing a broadcast channel multiple access
• link layer addressing
• various link layer technologies
• Ethernet (IEEE 802.3, CSMA/CD)
• switched LANS
• PPP
• virtualized networks as a link layer ATM, MPLS